## some CPUs enable their on-chip cache to be used temporarily as a scratch RAM (stack), e.g. cpu/amd/model_lx/cache_as_ram.inc

## some CPUs enable their on-chip cache to be used temporarily as a scratch RAM (stack), e.g. cpu/amd/model_lx/cache_as_ram.inc

−

# the final <code>mainboardinit</code> fragment is mainboard-specific, in 'C', called auto.c (or cache_as_ram_auto.c). It is compiled with 'romcc', and it includes and uses other C-code fragments for:

+

# the final <code>mainboardinit</code> fragment is mainboard-specific, in 'C', called romstage.c. For non-cache-as-RAM targets, it is compiled with 'romcc'. It includes and uses other C-code fragments for:

Introduction

This manual is intended for aspiring coreboot developers to help them get up to speed with the code base and the tasks required to add support for new chipsets, devices, and mainboards. It currently covers coreboot v2, but will be extended to also cover the development version coreboot v3 later.

Recommended hardware and software tools for developers

See Developer Manual/Tools for a list of recommended tools which are useful for coreboot users and developers.

Hardware Overview

Intel Architecture

Hardware Reset

The first instruction that is fetched and executed following a hardware reset is located at physical address 0xFFFFFFF0. This address is 16 bytes below the processor’s uppermost physical address. The EPROM containing the software-initialization code must be located at this address.

The address 0xFFFFFFF0 is beyond the 1-MByte addressable range of the processor while in real-address mode. The processor is initialized to this starting address as follows. The CS register has two parts: the visible segment selector part and the hidden base address part. In real-address mode, the base address is normally formed by shifting the 16-bit segment selector value 4 bits to the left to produce a 20-bit base address. However, during a hardware reset, the segment selector in the CS register is loaded with 0xF000 and the base address is loaded with 0xFFFF0000. The starting address is thus formed by adding the base address to the value in the EIP register (that is, 0xFFFF0000 + 0xFFF0 = 0xFFFFFFF0).

The first time the CS register is loaded with a new value after a hardware reset, the processor will follow the normal rule for address translation in real-address mode (that is, [CS base address = CS segment selector * 16]). To insure that the base address in the CS register remains unchanged until the EPROM based software-initialization code is completed, the code must not contain a far jump or far call or allow an interrupt to occur (which would cause the CS selector value to be changed).

coreboot Overview

View From The CPU: Intel Architecture

at 0xFFFFFFF0, start execution at reset_vector from src/cpu/x86/16bit/reset16.inc, which simply jumps to _start

_start from src/cpu/x86/16bit/entry16.inc, invalidates the TLBs, sets up a GDT for selector 0x08 (code) and 0x10 (data), switches to protected mode, and jumps to __protected_start (setting the CS to the new selector 0x08). The selectors provide full flat access to the entire physical memory map.

__protected_start from src/cpu/x86/32bit/entry32.inc, sets all other segment registers to the 0x10 selector

execution continues with various mainboardinit fragments:

__fpu_start from cpu/x86/fpu/enable_fpu.inc

(unlabeled) from cpu/x86/sse/enable_sse.inc

some CPUs enable their on-chip cache to be used temporarily as a scratch RAM (stack), e.g. cpu/amd/model_lx/cache_as_ram.inc

the final mainboardinit fragment is mainboard-specific, in 'C', called romstage.c. For non-cache-as-RAM targets, it is compiled with 'romcc'. It includes and uses other C-code fragments for:

initializing MSRs, MTTRs, APIC

setting up the Southbridge minimally ("early setup")

setting up SuperIO serial

initializing the console

initializing RAM controller and RAM itself

execution continues at __main from src/arch/i386/init/crt0.S.lb, where the non-romcc 'C' coreboot code is copied (possibly decompressed) to RAM, then the RAM entry point is jumped to.

the RAM entry point is _start arch/i386/lib/c_start.S, where new descriptor tables are set up, the stack and BSS are cleared, the IDT is initialized, and hardwaremain( ) is called (operation is now full 32-bit protected mode 'C' program with stack)

hardwaremain( ) is from boot/hardwaremain.c, the console is initialized, devices are enumerated and initialized, configured and enabled

the payload is called, either via elfboot( ) from boot/elfboot.c, or filo( ) from boot/filo.c

Serial output and the Super I/O

Northbridge

RAM init

Southbridge

Mainboard

Config.lb

The mainboard config.lb contains many build and platform configuration settings. One of the most important items is the mainboard device list.

A device needs to be listed in the mainboard config.lb if it requires more setup than standard PCI initialization (resource allocation). Typically, that includes the CPU, northbridge, southbridge, and SIO. These devices are usually required for system specific configuration as well as indicate the system bus structure (pci_domain).

When a device in config.lb is found during the coreboot PCI/system scan process the functions to do customized initialization are called via the device_operations and the chip_operations structures. You will find these structures in the devices source files.

Specific datasheets

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